I saw an interesting hexagonal ballpoint pen on Muji's website and was curious to see it in person, so off to a shop I went to have a look. But there were no metal pens like that in the store. What they did have was a ballpoint that looks like their aluminum fountain pen. Measuring about 5 1/3 inches when capped, the Mulji aluminum ballpoint has a needle point tip that produces a 0.7mm width line.
The aluminum ballpoint has one of the nicest oil-based inks I've used in some time, although there is some blotting so smearing is likely. It's a light-weight pen, with a brushed grip that is easy on the fingers and a shiny, polished metal barrel and capt.
The ink cartridge is one of the biggest I've seen, and surprising since Muji gel ink pens tend to use very think ink refills. The aluminum ballpoint is pricey ($16.50 plus tax), and may be in the phase-out stage as it's not on the website. It might also be an in-store only purchase, while the hexagonal ballpoint is online only. It's an interesting pen, and one that I'll be carrying with me for the times I need something other than a fountain pen to make a quick note.